Cooling arrangement for a tapered turbine blade

Development for this invention was supported in part by Contract No. DE-FC26-05NT42644, awarded by the United States Department of Energy. Accordingly, the United States Government may have certain rights in this invention.

This present invention relates to the field of turbine blades, and more particularly, the present invention relates to highly tapered and twisted turbine blades having internal cooling channels for passing cooling fluid to cool the turbine blades.

Gas turbine engines include a compressor for compressing air, a combustor for mixing the compressed air with fuel and igniting the mixture, and a turbine assembly for producing power. Combustors often operate at high temperatures that may exceed 2,500 degrees Fahrenheit. Typical turbine combustor configurations expose turbine assemblies to these high temperatures. As a result, turbine blades must be made of materials capable of withstanding such high temperatures. In addition, turbine blades often contain cooling arrangements for additional thermal protection.

Typically, turbine blades are formed from a root at one end for engaging a shaft and an elongated radial portion forming an airfoil that extends outwardly from a platform coupled to the root. The blade is ordinarily composed of a tip opposite the root, a leading edge, and a trailing edge. The interior structure of most turbine blades typically contains cooling channels forming part of a cooling arrangement. The cooling channels in the blades receive air from the compressor of the turbine engine and pass the air through the blade. The cooling channels often include multiple flow paths. Centrifugal forces and air flow at boundary layers may result in localized hot spots. Localized hot spots, depending on their location, can reduce the useful life of a turbine blade.

The cooling scheme for a turbine blade will depend upon its location within the turbine. The temperature of the working fluid will decrease as the fluid expands through the turbine and imparts its energy to the machine in the form of shaft power. Thus, the first row of blades is subjected to the highest gas temperature, and each successive row is subjected to a sequentially lower gas temperature. In addition, each successive row of blades gets longer in the radial direction, and may include more taper in cross-sectional area from root to tip, and may include more twist about its radial axis from root to tip. Row 1 blades of current generation industrial gas turbines are coated with a ceramic thermal barrier coating material and also include internal cooling fluid passages; whereas no ceramic coating material and no active cooling is needed for Row 4 blades of the same machines. U.S. Pat. No. 6,910,864 discloses a cooling scheme for a Row 2 industrial gas turbine blade consisting of a series of generally radially oriented cooling holes passing through the blade interior.

The invention is explained in the following description in view of the drawings that show:

FIG. 1 is a cross-sectional view of an industrial gas turbine blade in accordance with one embodiment of the present invention.

FIG. 2 is a partially sectioned top view of a cooled blade shroud that may be used with the blade cooling arrangement of the present invention.

The firing temperature of modern gas turbine engines continues to increase in response to the ongoing demand for improved energy efficiency. It is now desired to cool blades as far into the turbine as Row 4. The present inventor has recognized that known cooling arrangements for Row 2 turbine blades, such as U.S. Pat. No. 6,910,864 cited above, may be workable for some Row 3 blade designs, but they are not workable for typical fourth stage turbine blades because known manufacturing techniques for creating generally radially oriented cooling holes are not reliable for blades having a high degree of taper and/or twist. Furthermore, the present inventor has recognized that prior art radially oriented cooling schemes tend to provide a higher degree of cooling on the tip portion of a tapered blade because the cross-sectional area of the cooling flow paths represent a higher percentage of the airfoil cross-sectional area in the tip region than in the root region. The present inventor finds this to be counter-productive, because centrifugal loads are higher in the root portion of the blade, and therefore less material strength is available to accommodate thermal stress in the root portion than in the tip portion. The present inventor has thus endeavored to provide a cooling arrangement for highly tapered gas turbine blades that features an increased focus on cooling an inner radial portion of the turbine blade as compared with an outer radial portion of the turbine blade, and that can be implemented with known manufacturing techniques even when the blade is highly twisted.

Referring to the sole figure, a turbine blade 10 for Row 4 of an industrial gas turbine engine in accordance with one embodiment of the present invention will now be described. The turbine blade 10 includes a cooling arrangement 11 in inner aspects of the turbine blade for use in gas turbine engines. While the inventive cooling arrangement 11 is particularly well suited for a turbine blade 10 , the cooling arrangement 11 may also be used in a stationary turbine vane. The cooling arrangement 11 is especially advantageous for gas turbine blades 10 with a root-to-tip cross-sectional area ratio of 4:1 or higher. The cooling arrangement 11 illustratively includes dual triple-pass serpentine cooling circuits 80 , 82 formed in a lower radial portion 50 of the blade 10 , and receiving cooling fluid from a root portion 24 of the blade. The cooling arrangement 11 also includes a pair of single radial channel cooling circuits 84 , 86 formed in an upper radial portion 52 of the blade and receiving the cooling fluid from a respective one of the serpentine cooling circuit 80 , 82 . This arrangement provides a higher degree of heat removal capability in the inner radial portion of the blade when compared to the heat removal capability in the outer radial portion of the blade. A first 84 of the pair of radial cooling circuits 84 , 86 is disposed to cool an upper leading edge portion 15 of the blade, and a second 86 radial cooling circuit is disposed to cool an upper trailing edge portion 17 of the blade. The gas turbine blade 10 illustratively includes a leading edge 12 and a trailing edge 14 , and corresponding triple-pass serpentine cooling circuits 80 , 82 disposed to cool respective lower leading edge portion 13 of the blade and lower trailing edge portion 19 of the blade. The leading edge serpentine triple-pass cooling circuit 80 may have a higher heat load and/or average temperature than the trailing serpentine triple-pass cooling circuit 82 during operation of the gas turbine blade 10 . Various cooling parameters of each respective triple-pass serpentine cooling circuits 80 , 82 , such as mach number of cooling fluid through each respective triple-pass serpentine cooling circuit, number and/or type of turbulators, size dimensions, and flow rates may be adjusted as needed to counteract the respective heat loads and/or temperatures of each triple-pass serpentine cooling circuit during operation of the turbine blade 10 . Although the sole figure illustrates a respective triple-pass serpentine cooling circuit adjacent each of the lower leading and trailing portions 13 , 19 , other embodiments may feature additional serpentine structures and/or other than a triple-pass structure.

The turbine blade 10 further includes a generally concave pressure side (not shown) and a generally convex suction side (not shown) for coupling the leading edge to the trailing edge. More particularly, the turbine blade 10 includes a tip 20 at a first end 22 , and a root 24 at a second end 26 longitudinally opposite the first end. The root 24 has a lower surface 28 positioned opposite from the second end 26 . The turbine blade 10 may have a highly tapered shape, with the cross-sectional area of the airfoil proximate the second end 26 being much greater than the cross-sectional area of the airfoil proximate the first end 22 , such as with a ratio of at least 4:1 in various embodiments.

The turbine blade 10 further includes a serpentine cooling path 30 in its radially inner portion having a plurality of pairs of channels and pairs of turns, where each pair of turns couples consecutive pairs of channels together. The plurality of pairs of channels include a pair of inflow channels 32 , 34 longitudinally extending from the root lower surface 28 between the leading edge 12 and the trailing edge 14 through the second end 26 and to a pair of inflow turns 36 , 38 at an intermediate height 40 between the first and second end 22 , 26 . As shown in the exemplary embodiment of the figure, the pair of inflow channels 32 , 34 may extend longitudinally from the root lower surface 28 at approximately the midpoint between the leading edge 12 and trailing edge 14 , although in other embodiments the pair of inflow channels may extend longitudinally from the root lower surface 28 at any region between the leading edge 12 and trailing edge 14 . The turbine blade 10 further includes cover plates 76 for covering the lower surface 28 adjacent to the leading edge 12 and the trailing edge 14 . The lower surface 28 includes a pair of openings continuous with the pair of inflow channels 32 , 34 . As shown in the figure, the cover plates 76 do not block these openings to the pair of inflow channels 32 , 34 .

As illustrated in the figure, the plurality of pairs of channels include a pair of intermediate channels 42 , 44 extending longitudinally from the pair of inflow turns 36 , 38 to a respective pair of root turns 46 , 48 adjacent to the second end 26 . The pair of root turns 46 , 48 extend from the pair of intermediate channels 42 , 44 into the root 24 adjacent the second end 26 . The plurality of pairs of channels further include a pair of outflow channels 47 , 49 extending respectively adjacent to the leading edge 12 and the trailing edge 14 from the pair of root turns 46 , 48 to adjacent a radial position 54 between the first and second ends, as discussed below. The root turns 46 , 48 feature less aerodynamic weight, are easier to cast, and include less overall weight than prior art turns in serpentine cooling arrangements. Each root turn 46 , 48 may function as a manifold by receiving an outlet of a respective first intermediate channel 42 , 44 and simultaneously feeding the cooling fluid to the respective outflow channel 47 , 49 , thereby providing less aerodynamic pressure loss than a prior art U-bend turn. Although the figure illustrates one pair of inflow channels, one pair of intermediate channels, one pair of outflow channels, as well as one pair of inflow turns and root turns, other embodiments may include other serpentine arrangements in the radially inner portion of the blade to provide a high degree of cooling for this highly stressed portion of the blade.

The pair of inflow turns 36 , 38 is positioned in a inner (lower) radial portion 50 of the turbine blade proximate a radial position 54 between the first 22 and second end 26 of the blade airfoil. An associated outer (upper) radial portion 52 of the turbine blade is positioned between the radial position 54 and the first end 22 . The radial position 54 may be positioned at the midpoint between the first and second end 22 , 26 , or it may be positioned above or below such midpoint, as is necessary to focus the cooling of the serpentine cooling paths that exist below such radial position. Selection of the location of radial position 54 is a design variable that may be manipulated as appropriate to account for the pattern of stresses encountered in the turbine blade. In the illustrated embodiment, the ratio of the number of channels positioned within the lower radial portion 50 of the turbine blade to the number of channels positioned within the upper radial portion 52 of the turbine blade is at least 2:1, and preferably 3:1, as is illustrated in the sole figure, with three pairs of channels ( 32 , 34 ) ( 42 , 44 ) ( 47 , 49 ) within the lower radial portion 50 and one pair of channels ( 84 , 86 ) within the upper radial portion 52 . The effectiveness of the cooling provided by each channel is a function of the size of the channel, which in turn, is a function of the number of parallel channels that exist across the airfoil cross-section. Furthermore, the cooling fluid average temperature will be lower within the serpentine channels of the radial inner portion of the blade than in the radial channels of the radial outer portion of the blade, thereby further focusing the cooling capacity onto the most highly stressed portion of the blade.

Cooling fluid is directed through the serpentine cooling path 30 , which is typically air received from a compressor (not shown), through the turbine blade 10 and out one or more exit orifices adjacent the first end 22 . In the exemplary embodiment of the sole figure, the cooling fluid flows through the serpentine cooling path 30 and the pair of adjacent inflow channels 32 , 34 in a common direction. The cooling fluid is passed through the serpentine cooling path 30 , including the pair of inflow channels 32 , 34 and the pair of intermediate channels 42 , 44 in the lower radial portion 50 of the turbine blade, after which the used cooling fluid is passed to the upper radial portion 52 of the turbine blade and outputted through exit orifices adjacent to the first end 22 .

A plurality of ribs 56 positioned within the blade interior structure separate and defines the consecutive channels. The pair of root turns 46 , 48 extend into the root 24 from the pair of intermediate channels 42 , 44 . The pair of root turns 46 , 48 include a pair of open root turns 58 , 60 within respective root cavities 62 , 64 . Each respective root cavity 62 , 64 is defined by a root pressure side and a root suction side opposite from the root pressure side. Additionally, each respective root cavity 62 , 64 is further defined by a rib portion 69 extending into the root 24 to the lower surface 28 between each intermediate channel 42 , 44 and a respective adjacent inflow channel 32 , 34 . Each open root turn 58 , 60 may form a free stream manifold for cooling fluid flowing through the serpentine cooling path 30 .

As illustrated in the sole figure, a portion 57 of a rib 56 between the pair of intermediate channels 42 , 44 and at one end of the pair of outflow channels 47 , 49 may be arcuate. The arcuate portion of the rib between the pair of intermediate channels and the pair of outflow channels may be positioned at an intermediate height 40 to accommodate the transition from a higher number of channels in the inner radial portion to a lower number of channels in the outer radial portion for focused cooling of the lower radial portion of the turbine blade.

The turbine blade 10 may further include a first end shroud 70 adjacent to the first end 22 . The first end shroud 70 includes a plurality of exit orifices 72 along the first end 22 for providing a plurality of outlets for used cooling fluid having passed through the turbine blade.

The channels may include a plurality of turbulators or trip strips 74 positioned along the sides of the channels in order to enhance mixing and cooling efficiency. The number, spacing, size and location of the trip strips 74 may be selected to optimize the degree of cooling achieved in the inner and outer radial portions of the cooling arrangement.

The turbine blade 10 may be manufactured by known casting techniques. Two halves of the turbine blade 10 may be manufactured separately and then joined along a cord line by any known joining technique, such as transient liquid phase bonding for example.

Operation of the cooling arrangement 11 provides two generally parallel cooling fluid flows through the blade interior, one through the leading edge portion and one through the trailing edge portion of the blade. Various cooling parameters such as channel size, number of channels, turbulators, etc. may be varied between these two parallel flows to optimize the cooling provided for the leading edge portion and the trailing edge portion. During operation, the cooling fluid flows through the openings in the lower surface 28 of the root 24 , into the pair of inflow channels 32 , 34 and to the pair of inflow turns 36 , 38 adjacent an intermediate height 40 within a lower radial portion 50 of the turbine blade between the first end 22 and second end 26 . As illustrated in figure, cooling fluid flowing through inflow channel 32 and to inflow turn 36 turns down into the intermediate channel 42 before entering the root turn 46 , and an open root turn 58 extending from the intermediate channel 42 into a root cavity 62 within the root 24 . Upon passing through the root turn 46 , the cooling fluid enters the outflow channel 47 adjacent the leading edge 12 . The cooling fluid enters a single radial channel cooling circuit 86 at an arcuate portion 57 of the rib adjacent the intermediate height 40 and between one end of the outflow channel 47 and the intermediate channel 42 . The cooling fluid then passes through a plurality of exit orifices 72 in shroud 70 upon reaching the first end 22 and is discharged from the turbine blade 10 . Based on the similar structure of the other pair of cooling flow channels, cooling fluid passing through the inflow channel 34 is similarly routed through the turbine blade 10 after it passes through the inflow channel 32 , with the exception that the inflow turn 38 turns the cooling fluid toward the trailing edge 14 , and thereby eventually causes the cooling fluid to pass through the single radial channel cooling circuit 86 adjacent the trailing edge 14 .

FIG. 2 illustrates a cooled blade shroud 90 such as may be part of a cooling arrangement for an industrial gas turbine blade of the present invention. The cooled shroud 90 may be used in lieu of the shroud 70 on the blade illustrated in FIG. 1 . The shroud 90 includes a tip rail 91 formed opposed the airfoil portion of the blade (not shown). The shroud includes a pair of cooling channels 92 , 94 formed internal to the shroud and illustrated in phantom. The cooling channels 92 , 94 are adapted to receive the cooling fluid exhausted from radial channel cooling circuits 84 , 86 respectively, when used on the blade of FIG. 1 . A plurality of cooling holes 96 (also illustrated in phantom) are in fluid communication with the cooling channels 92 , 94 for passing the cooling fluid through the body of the shroud 90 to respective exit points around the perimeter of the shroud 90 . A portion of the shroud 90 is illustrated in a cut-away view to reveal laminar flow disruptors that optionally may be incorporated into some or all of the cooling holes 96 in order to improve heat transfer between the cooling fluid and the shroud 90 . The flow disruptors may take the form of a plurality of turbulators 98 formed along the length of the hole 96 or a single helical rib 100 extending in a spiral pattern along the length of the hole 96 , for example.

While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.